This invention relates to frequency modulated signal sources. More particularly, this invention relates to signal sources for supplying a frequency modulated signal at any one of numerous selectable carrier frequencies wherein the signal source includes a frequency discriminator that is tuned to the selected frequency and connected to supply a negative feedback signal that stabilizes and/or reduces the noise level of the signal source.
As is known in the art, various circuit arrangements have been proposed and utilized to supply a frequency modulated electrical signal. Regardless of the exact techniques utilized to generate the carrier signal and the circuit structure involved, the basic design objective is to realize an overall circuit arrangement which exhibits a linear relationship between the magnitude of the modulating signal and the frequency deviation caused by that signal (i.e., .DELTA.f/v=k; where v is the instantaneous value of the modulating signal, .DELTA.f is the difference between the resulting frequency and the carrier frequency and k is a constant). With respect to a fixed frequency sinusoidal carrier signal having a radian frequency of .omega..sub.c and a sinusoidal modulating signal having a radian frequency of .omega..sub.m, such a relationship results in the well-known frequency modulation expression v.sub.o =A cos (.omega..sub.c +M.sub.f cos .omega..sub.m t)t, where A is a constant and the modulation index or deviation ratio M.sub.f is equal to the maximum frequency deviation divided by the frequency of the modulating signal.
In many situations a signal source must provide a frequency modulated signal at a relatively large number of carrier frequencies that extend over a substantial frequency range and the problems associated with achieving a circuit design that satisfactorily realizes the above-noted linear modulation characteristic are greatly increased. In particular, inherent nonlinearities in the modulation characteristics associated with practical circuit realizations cause distortion of the modulated signal. When a large number of carrier frequencies must be accommodated, achieving an acceptable distortion level not only requires that the system modulation characteristic be substantially linear but also requires that the modulation characteristic be substantially invariant relative to the set of carrier frequencies of interest. Because all components and networks exhibit at least a certain degree of frequency dependence, prior art signal sources that provide satisfactory (eg, low-distortion) frequency modulation over a relatively wide range of carrier frequencies are often relatively complex.
Additional design constraints often compound the above-noted problems of designing relatively broadband, frequency modulated signal sources. For example, signal sources that serve as laboratory instrumentation and signal sources utilized in channelized communication equipment often must supply frequency modulated signals wherein the carrier frequency is a precise, selected value. Such signal sources must not only additionally exhibit substantial long-term and short-term frequency stability, but must often meet demanding noise requirements with respect to stochastic internal circuit noise and coherent noise such as that caused by nonlinear circuit operation and bias supply ripple. Moreover, remote programming or signal selection via a digitally encoded or analog control signal is often a desirable or necessary feature.
Prior art attempts to provide frequency modulated rf signal sources basically include circuitry wherein the modulating signal directly controls the operating frequency of various types or classes of tuned oscillators and arrangements wherein the carrier frequency is produced by a conventional oscillator arrangement and frequency modulation is induced by subsequent processing of the carrier signal. Generally, those circuits which directly control the frequency of a tunable signal source include variable reactance devices which are electrically coupled to the resonant circuit that determines the frequency of oscillation wherein the modulating signal is coupled to the variable reactance and thereby controls the signal frequency. For example, in one type of prior art arrangement the frequency of an active oscillator circuit is established by a mechanically-tuned resonant cavity and frequency modulation is induced by supplying the modulating signal to a varactor diode that is coupled to the resonant cavity. Since the coupling between the varactor diode and the resonant cavity does not detrimentally affect the relatively high Q of the cavity, this type of arrangement exhibits a relatively high signal to noise ratio and permits selection of precise carrier frequencies. However, selection of each desired carrier frequency requires manual adjustment of the mechanically-tuned cavity and concomitant adjustment of the electrical coupling between the varactor diode in order to provide a relatively constant modulation characteristic at each frequency of interest. Thus, such an arrangement may require fairly complex mechanical tuning devices. Further, because of the mechanical tuning requirement, it may not be possible to change such a signal source between accurately defined carrier frequencies as rapidly as desired or necessary and remote frequency programming is often either impractical or requires a complex control mechanism.
Arrangements in which a selectable frequency carrier signal is generated by a conventional variable frequency oscillator and frequency modulated within subsequent circuit stages are typified by systems in which a signal at the desired carrier frequency is supplied by a first signal source and mixed or heterodyned with a frequency modulated signal that is obtained by modulating a second signal source which operates at a substantially invariant center frequency. Although the heterodyned signal is band-pass filtered, the necessary nonlinear characteristic of the mixing stage of such a system causes the generation of substantial spurious signal components. Further, systems of this type that provide carrier frequencies extending over a relatively large frequency range (eg, one octave or more) are relatively complex and costly.
In situations that require remote frequency programming to enable selection of a desired carrier frequency from a large number of signal frequencies, signal sources that are commonly referred to as frequency synthesizers are often employed and several arrangements have been proposed for frequency modulating various types of frequency synthesizers. One type of frequency synthesizer that has found widespread application is known as a phase-locked loop system includes a voltage controlled oscillator (VCO) that is phase-locked to a reference signal to thereby cause the VCO to operate at a frequency that is a rational, mathematical function of the reference signal frequency, e.g., f.sub.out =Nf.sub.ref, where N is a selectable integer that is established by a programmable frequency divider that forms a portion of the phase-locked loop feedback network and f.sub.ref is the of the reference signal.
Since most phase-locked loop networks are designed to exhibit a relatively wide bandwidth and the phase-locked loop operates to eliminate phase and frequency perturbations occurring at rates within the loop bandwidth, it has been necessary to develop various circuit arrangements for satisfactorily phase or frequency modulating phase-locked systems. One approach to frequency modulating such a system in effect combines two modulation paths, the first of which accommodates fm rates within the bandwidth of the phase-locked loop and the second of which accommodates fm rates greater than the phase-locked loop bandwidth. In one system of this type, modulation signals at fm rates within the bandwidth of the phase-locked loop modulate the oscillator that supplies the reference signal and modulation signals at fm rates outside of the bandwidth of the phase-locked loop are coupled to the frequency control terminal of the phase-locked loop voltage controlled oscillator. A second arrangement of this type also utilizes frequency modulation of the phase-locked loop voltage controlled oscillator for modulation signals outside the phase-locked loop bandwidth and accomodates fm rates within the phase-locked loop bandwidth by phase modulating the phase-locked loop phase detector. In this type of arrangement, modulation signals at rates less than the crossover frequency of the phase-locked loop are typically coupled to the input terminal of the loop filter via a conventional integrator circuit.
Although properly compensating the above-described arrangement to equalize the signal delay in the two modulation paths often provides satisfactory circuit operation, it is often difficult to obtain the previously mentioned linear, frequency invariant modulation characteristic over a desired range of carrier frequencies. Further, because the stochastic phase noise generated by state of the art voltage controlled oscillators is substantially higher than that of frequency modulated oscillators employing high Q resonant networks (e.g., a resonant cavity), prior art frequency modulated phase-locked loops have not satisfied the extremely demanding performance requirements such as those associated with high quality channelized communication systems and laboratory test equipment.
In this latter regard, the low noise signal source disclosed in a U.S. patent application of Donald G. Meyer, entitled "Controlled Frequency Source Apparatus Including a Feedback Path for the Reduction of Phase Noise", Ser. No. 168,065 filed of even date with this application and assigned to the assignee of this invention provides a phase-locked loop having remote signal selection capabilities, a frequency range of one octave or more, and substantially less phase noise than previously realizable phase-locked loop systems. However, the frequency modulation problems associated with more conventional prior art phase-locked loop frequency synthesizers are not overcome or alleviated by the arrangement disclosed in the referenced patent application by Meyer, but, in fact, become more complex. In particular, the low noise programmable divide-by-N phase-locked loop arrangements disclosed in the Meyer application not only include a feedback path for establishing and maintaining phase-lock at the selected signal frequency, but also include a second feedback path that reduces phase noise by, in effect, demodulating the VCO output signal and supplying negative feedback that is proportional to VCO phase noise to the VCO frequency control terminal. This additional feedback path includes a frequency discriminator of the type which includes a time delay network (e.g., a surface acoustic wave (SAW) delay device or a coaxial cable) that is serially interconnected between one input port of a phase detector and the output terminal of the VCO. Variable phase shifting apparatus, included in one or both of the phase detector input paths, is automatically controlled in response to the signal provided by the phase detector and/or the system frequency selection signal so as to maintain the phase detector output signal substantially equal to zero at each selected frequency of operation (i.e., at phase-lock). Thus, the frequency discriminator effectively tracks the phase-locked loop to provide negative feedback to the VCO frequency control terminal and, by properly configuring the system, low phase noise is attained without significantly altering frequency selection (tuning) characteristics. However, high quality, low-distortion frequency modulation of such a signal source at each selected frequency over a relatively wide frequency range is inherently difficult in such a system because ideal, frequency invariant amplitude and phase (delay) characteristics cannot be attained in either the feedback path for maintaining phase-lock or the feedback path for reducing phase noise. In fact, since the amplitude and delay characteristics of stepped phase shifters and switched delay networks that are used in some embodiments of the system disclosed by Meyer are discontinuous functions of frequency, the phase-locked loop signal sources disclosed in the referenced Meyer patent application are not capable of accurate frequency modulation by direct application of prior art techniques.
Accordingly, it can thus be recognized that the prior art has provided numerous frequency modulated signal sources but, prior to this invention, has not provided a signal source which simultaneously satisfies design objectives and constraints associated with the more demanding signal source requirements. As previously mentioned, such signal source requirements include accurate, low-distortion phase or frequency modulation of numerous selected carrier frequencies that extend over a relatively wide frequency range, low noise, electronic tuning (frequency programming) and high reliability.
In the broadest sense it is thus an object of this invention to provide a frequency modulated signal source which permits remote selection of a single carrier frequency from numerous carrier frequencies that extend over a relatively wide bandwidth.
It is another object of this invention to provide a phase-locked loop signal source which can be frequency modulated at each of the synthesized signal frequencies.
It is yet another object of this invention to provide an improved phase-locked loop frequency synthesizer for supplying a low-noise frequency modulated signal.
Still further, it is an object of this invention to provide a means for accurate frequency modulation of a low-noise, phase-locked loop frequency synthesizer of the type disclosed in the previously-referenced patent application by Meyer.